The industries and agencies responsible for aviation, space exploration, and defense are not satisfied with being restricted to supersonic speeds. Rather, reusable vehicles are being designed to permit hypersonic suborbital and earth-to-orbit missions.
As Mark Opeka of the US Naval Surface Warfare Center explained to me, generally, a Mach number of five distinguishes the supersonic from the hypersonic regime. This differentiation has its origins in wind-tunnel testing. Above a Mach number of ∼5, a typical diatomic working fluid (air or N2) must be preheated to prevent condensation occurring in the test cell where the test object (or model) is located. Obviously, condensation in a wind tunnel would result the model being pelted with high velocity liquid, which is usually not desirable.
The requirement to preheat the working gas is not a trivial one. In a state-of-the-art wind tunnel, a graphite resistance heater is used to preheat N2 to 2200°C under 1500 atm. pressure. The preheated gas is driven though a converging-diverging nozzle into the test cell and empties into a huge (25 m diameter) evacuated sphere. In this way, a full-size reentry body can be tested at realistic Mach numbers and surface pressures for a 1-2 s run time. For specific hypersonic applications such as scramjet combustors, wing leading edges, rocket nozzles, and nose cones, materials must retain their form and strength at temperatures of 2000-2400°C in the presence of oxidizing combustion product gases or air. No simple alloy or ceramic composite can satisfy these conditions while avoiding melting, oxidation, and evaporation. Therefore, oxidation/evaporation-resistant coatings are required for a high-melting substrate with adequate strength but inadequate oxidation resistance.
The starting point is naturally provided by the material scheme used successfully for the surfaces of the space shuttle the most severely tested upon reentry, namely carbon-fiber-wound parts held in an amorphous carbon matrix protected from oxidation/evaporation by a SiC conversion (diffusion) coating. This system is reliable to 1600°C barring any destructive impact by debris or meteorites. To improve on this performance base, C-C, C-SiC, and SiC-SiC composite substrates are gaining attention, along with ultrahigh-temperature ceramics such as Zr(Hf)B2-SiC.
Henry Graham at Wright-Patterson Air Force Base showed long ago that no monolithic ceramic could have enough fracture toughness to stop crack propagation. So composites are the only game in town. But composite substrates necessarily exhibit a different thermal expansion coefficient from any candidate surface coating. So, under cyclic oxidizing service conditions, the coating will need to close up surface cracks to prevent exposure of the substrate, e.g. a C-C composite. This condition dictates that the oxidation- and evaporation-resistant protective product phase must be a pliable, adherent, glassy barrier oxide, namely silica. While the open structure of silica allows some inward diffusion of molecular oxygen, other oxidation products (SiO, CO, N2) can also diffuse outward through the glass without failure of the protective oxide film. The purpose of Zr or Hf in the substrate or the coating is to form ZrO2 or HfO2 in a morphology that minimizes the glassy cross-section and increases the glass viscosity while decreasing its diffusivity for molecular oxygen. Of course, one needs to worry whether the ZrO2 or HfO2 phases themselves serve to permit oxygen ion diffusion resulting from their electronic components to electrical conduction. The intended function of B in the ceramic is to permit the formation of a crack-filling glassy phase at a lower temperature. Boron oxide would be lost to evaporation at the highest temperatures, especially in the presence of water vapor, since hydrated vapor species are inherent to boron oxide.
Because of the horrific expense of wind-tunnel testing and the significant expense of arc-heater testing, most preliminary laboratory oxidation studies of candidate materials are performed at the correct temperature, but in the presence of slow air convection. Depending on the intended application, the disparity of the test gas to that in the service application can be decisive, especially since the evaporation rate and mechanical shearing of the silica glass will depend on the composition of the gas and its impact velocity. Even the microstructure, e.g. void fraction and distribution, of the protective scale is changed upon changing the test conditions for the same temperature. Thus, modeling of the oxidation kinetics must be done under different assumptions, depending upon the experimental conditions.
Just when one might suppose that the world's technology has reached a limit in how fast, how hot, or how far engineering systems will go, goals and needs arise for further advances that are invariably limited by the development of improved materials.